Triggering Muscle Growth

When it comes to muscle, protein is king. Although water makes up the majority of muscle tissue (approximately 70 percent of muscle weight), it is the protein component (approximately 25 percent of muscle weight) that is responsible for carrying out human movement. The extent of muscle development is predicated on the balance between muscle protein synthesis, or building, and muscle protein breakdown. When synthesis is greater than breakdown, you are in an anabolic (building up) state that’s conducive to building muscle.

Contrary to popular belief, you don’t build your muscles when you work out. In fact, the opposite actually occurs. Muscle tissue breaks down at an accelerated rate during training, and protein synthesis is largely suppressed. Although this may seem counterintuitive, it is necessary for facilitating bigger, stronger muscles. Compare it to renovating your kitchen. You have to tear out the existing Formica countertops and the pressboard cabinetry before installing the high-end granite and fine hardwood, right? Similarly, outmoded muscle proteins must first be demolished and removed in order to allow newer, better proteins to take their place.

Muscle tissue rebuilds after exercise. During this time, muscle protein synthesis skyrockets and breakdown gradually diminishes. Protein synthesis can remain increased for 48 hours or more postexercise. During this time your muscles supercompensate by growing larger.

The underlying processes responsible for muscle development are highly complex and not well understood. It is generally accepted that the regulation of muscle tissue is carried out, at least in part, through the signaling of various pathways associated with protein synthesis and breakdown. These pathways are diverse and provide a variety of ways for muscle to adapt to overload. The common element of all muscle-building pathways is that they conduct signals through specialized enzymes, setting off a chain of events that ultimately promote protein synthesis and inhibit protein breakdown.

Current research indicates that three primary mechanisms are involved in exercise-related muscle growth: muscle tension, muscle damage, and metabolic stress (Schoenfeld 2010).

Muscle tension. Tension exerted on muscles during resistance exercise is generally considered the most important factor in muscle development. The tension from lifting weights disturbs the integrity of working muscles, thus bringing about a phenomenon called mechanotransduction. Simply stated, mechanotransduction is the process by which mechanical signals are converted into chemical activity; in this case, the signals turn on anabolic pathways. Up to a certain point, greater muscle tension leads to a greater anabolic stimulus—a classic case of adaptation. However, it seems that an upper limit exists, beyond which high tension levels have a diminishing effect on muscle growth. Once this threshold is reached, other factors become increasingly more important in the growth process. This is why bodybuilders generally display superior muscle growth compared with powerlifters even though bodybuilders routinely train with lighter weights.

Muscle damage. Anyone who lifts weights has undoubtedly felt achy and sore after an intense exercise session. This phenomenon, called delayed-onset muscle soreness (DOMS), generally manifests approximately 24 hours after an intense workout, and the peak effects are seen about two to three days postexercise. DOMS is caused by localized damage to muscle tissue in the form of microtears in both the contractile proteins and surface membrane (i.e., sarcolemma) of the working muscles. What many people fail to grasp, however, is that a certain amount of soreness may indirectly benefit muscle development. Here’s why: The response to muscle damage can be likened to the acute inflammatory response to infection. Once the body perceives damage, immune cells (neutrophils, macrophages, and so on) migrate to the damaged tissue in order to remove cellular debris to help maintain the fiber’s ultrastructure. In the process, the body produces signaling molecules called cytokines that activate the release of growth factors involved in muscle development. In this roundabout way, localized inflammation—a source of DOMS—leads to a growth response that, in effect, strengthens the ability of muscle tissue to withstand future muscle damage. Adaptation!

That said, soreness is by no means a prerequisite for muscle development. Your muscles, connective tissue, and immune system become increasingly efficient in dealing with fiber-related damage associated with intense training (again, an adaptive response). Various physiologic and structural adaptations that take place gradually reduce the sensation of pain. Generally speaking, the more you train at high levels of intensity, the greater your resistance to muscle soreness—even though you invariably inflict damage to fibers. This is why some of the world’s top bodybuilders never get sore after a workout, yet display impressive muscularity. Furthermore, too much soreness can actually be detrimental to muscle growth. If you’re so sore that it hurts to sit or comb your hair, you’ve exceeded your body’s ability to repair the damaged muscle tissue—and that means you’re not growing!

Metabolic stress. Perhaps the most intriguing factor associated with muscle development is exercise-induced metabolic stress. Research on patients confined to bed rest show that metabolic stress induced by the application of a pressure cuff can help attenuate muscle wasting, even in the absence of exercise. Other studies have found that pressure-cuff exercise performed with very light weights—far less than what is normally considered sufficient for promoting muscular adaptations—can promote significant muscle growth as a result of generating a substantial amount of metabolic stress.

The muscle-building effects of metabolic stress can be attributed to the production of by-products of metabolism called metabolites. These small fragments (including lactate, hydrogen ion, and inorganic phosphate) indirectly mediate cell signaling. Some scientists believe that this is accomplished by increasing water within muscle—a phenomenon known as cell swelling. Studies have shown that cell swelling stimulates protein synthesis and simultaneously decreases protein breakdown. It is not clear exactly why cell swelling causes an anabolic effect, but the prevailing theory suggests a self-preservation mechanism. That is, an increase in water within the cell exerts pressure against the cell wall, similar to overinflating a rubber tire. The cell, in turn, perceives this as a threat to its integrity and responds by sending out anabolic signals that initiate strengthening of its ultrastructure. Adaptation!

Understand that muscle tension, muscle damage, and metabolic stress generally do not exist in isolation. Rather, they combine to produce an additive effect on building lean muscle. Only by achieving an optimal mix of these factors in your training routine can you maximize muscle development.